It has first of all been suggested to produce these soles by starting with polymers in the form of granules which are extruded in the form of tape by means of a conventional process, particularly by means of a single- or twin-screw extruder, followed by a calender whose rolls have an adjustable gap. The process involved is very simple and quite economical, but has the disadvantage of resulting in soles whose abrasion resistance is limited with time.
To overcome these disadvantages it has been suggested, therefore, to employ high density polyethylene, no longer in the form of granules but of powder, that is to say having a very high molecular weight (higher than 5.times.10.sup.5, preferably higher than 1.times.10.sup.6). Unfortunately, this polymer cannot be extruded in a conventional manner in an extruder of the screw and barrel type because its very high melt viscosity requires motors of high power and oversized barrels which are difficult to control in respect of temperature. This results in selfheating of the polymer, which then rapidly degrades.
It has therefore been suggested to sinter this polymer, particularly by compacting the powder in a cylindrical mold, followed by pressing at a high pressure for an appreciable time at elevated temperature. The pancake thus obtained is then demolded and sliced with unwinding to obtain a polyethylene sheet of appropriate thickness which is rubbed down and then flamed to make it receptive to adhesive bonding. The thickness of the pancake corresponds to the width of the tape to be obtained. The slip and abrasion resistance properties are markedly improved in this manner. However, this process is time-consuming, requires repetitions and consequently results in costly products.
The invention overcomes these disadvantages. Its objective is a process for the manufacture of ski soles of high density polyethylene which makes use of a continuous extrusion process, and which is therefore economical and which does not degrade the properties of the material, thus retaining the qualities of the sole made of high density polyethylene of high molecular weight which is usually obtained by sintering.